This invention relates to the separation of solid-liquid or liquid-liquid suspensions employing inclined settling surfaces by either gravity sedimentation or floatation. In sedimentation, two of the basic design constraints are: the detention time within the settling chamber must be equal to or in practical application, greater than the time required for the denser component to settle to the floor of the chamber; and the means employed to remove the settled component from the chamber floor must minimize re-entrainment of the settled component.
The first condition can be satisfied by the use of a simple tank the volume of which is great enough to provide the required detention time as, for instance in U.S. Pat. Nos. 2,118,157 and 2,205,199. Such a design, however, is inefficient in that the ratio of throughput volume to detention volume is low. The efficiency can be greatly improved by installing a number of plates within the detention tank, disposed one above the other and generally parallel to each other and to the horizontal. Thereby each plate functionally provides a settling chamber. Such an arrangement is disclosed for instance, in U.S. Pat. Nos. 1,248,374 and 1,135,997. The increased efficiency of the design primarily results from the decreased distance the denser component must travel before settling out from the liquid being clarified.
Upon settling to the chamber floor the denser component can be considered removed from the flow stream since a velocity profile viewed along the direction of the flowing stream and from the side of the chamber shows a maximum along the center line of the chamber and zero at the upper and lower boundries (plates) of the chamber. Thus settled material normally is not subjected to sufficient force (from the flowing fluid) to cause it to re-enter the flow stream (re-entrainment).
Horizontally disposed plates possess the highest theoretical efficiency. However, in practice the value is considerably lowered by the actions of the various mechanisms employed to remove the settled material from the plates. Such mechanisms usually employ mechanical or hydraulic scrapers which cause considerable re-entrainment. Also in some designs the flow must be stopped for the duration of the removal process.
The practical efficiency of parallel plate separators can be increased by inclining the plates to the horizontal at some suitable angle. This inclination allows the force of gravity to cause the settled material to slide down and off the plates and into a collecting chamber or plenum. U.S. Pat. Nos. 3,706,384, 3,928,209, and 3,552,554 are examples of such devices. The first two are upflow sedimentation units, i.e. the suspension is injected into the bottom of the chamber containing the plates and flows upwardly while the settled material flows downwardly. The third is an example of a "downflow" sedimentation unit in which both the suspension and the settled material flow in a downwardly direction.
The upflow units illustrate two different approaches to injection of the suspension into the settling chamber. In both devices the injected fluid rapidly fans out to occupy the entire flow area within the chamber. The region between the point or points of injection and the location where the flow profile is completely developed is termed the transition region. It can be shown, both from hydraulic theory and from experimental observation, that transient turbulent eddys continuously form within this transition region. In order to be removed from the settling chamber, the settled material, which slides down the plates, must travel through this transition region and is thus subject to a degree of re-entrainment caused by interaction with the turbulent eddys. The tendency for re-entrainment to occur is increased by the increasing thickness of the settled material due to accumulation as the bottom of the flow chamber is approached. This causes a decrease in the area available for the upflowing fluid and results in an increased upflow velocity and an increase in the number and magnitude of turbulent eddys. These difficulties may be overcome by: increasing the height of the settling chamber; decreasing the upflow velocity; restricting the unit to suspensions with relatively low suspended material volumes. These minipulations all result in a unit possessing lowered efficiency.
The downflow unit mentioned above removes the settled material by causing the material to pass between the plates over which it is traveling and a clarified liquid collecting channel mounted on and projecting downwardly from the overlying plate of the chamber. Hereinafter, the passage between the lower plate and the collecting channel will be called the settled material collecting trough or "trough."
The settled material within the trough exhibits a great tendency to "bridge", i.e. the height of the settled material layer equals the height of the trough. Then frictional forces greatly retard further downward movement. Since the height of the material is rarely uniform across the width of the settling plates (due to turbulent eddys adjacent the inlet) portions of the trough are frequently bridged. Such bridging causes material to pile up behind the blockage and results in re-entrainment which is especially critical since it occurs adjacent to the effluent collector. The tendency to bridge can be reduced by: increasing the height of the trough, which in practice amounts to increasing the height of the settling chamber; increasing the downflow velocity, to increase the momentum of the settled material entering the trough; or increasing the angle of inclination of either the entire settling chamber or of that portion in the vicinity of the trough. These methods all result in units with decreased efficiency.